This happened because we erroneously redirect you to a page that was a 404 (at that time). We now won't redirect you in this condition.
We're switching some authentication paths to SSL (global auth in this case), that caused an unexpected overload of our nginx server running on the primary load balancer. Across the network we get a lot of requests for global auth (over 2 million a day)...turns out our configuration wasn't quite prepared for that amount of load (specifically, the socket pool ran dry pretty quick).
The short-term fix for that is simple: lower the load, in this case that meant disabling global auth on Stack Overflow (it accounts for ~1.5million of those ~2.2 million requests per day). The problem here? Disabling global auth didn't disable that specific logout redirect, that piece has been corrected.
We've now adjusted the SSL configuration we're running (as we'll continue too) and global auth is up and running again. We'll be keeping an eye on it all day just in case...if something goes very wrong you may notice global auth on SO disabled again, but no erroneous redirects.